1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image on the original.
2. Related Background Art
The applicant of the present invention has proposed the image reading apparatus used for a digital copying machine, an image reader a facsimile or the like in U.S. Pat. No. 4,864,416, U.S. Application No. 917,549, now U.S. Pat. No. 4,992,888, U.S. Pat. No. 4,663,672 etc. Some of these apparatuses employ the so-called optical system which scans the original placed on the glass of the original table by a light source and a mirror, and forms its image on the read sensor. As illustrated in FIG. 1, the said optical system illuminates the original P placed on the fixed glass 100 of the original table by a rod-shaped light source 101, reflects upon the required light of the image, which is the reflection of original P, by mirrors 102, 103 and 104 and further focuses the light of the image on the read sensor 106 such as a one-dimensional CCD image sensor, through the image forming lens 105.
In this case, the length of the light path extending from the surface of the original to the image forming lens 105 is always kept constant by setting the scanning speed of mirror 103 and 104 in the direction of arrow Q at a speed equal to one-half of the scanning speed in the direction of arrow Q of light source 101 and mirror 102.
Among this kind of image reading apparatus, there are some which require repeated interruption/resumption of reading whenever necessary during reading of one page of the original for convenience of the recipient of data. For example, in the case of an image scanner for computer input, when the buffer of the host computer for temporary storage of received data becomes full, the temporary reading by the image scanner is suspended and reading is resumed when the data processing by host computer is completed and input becomes possible.
That is, the image scanner shown in FIG. 1 sends the image data to the host computer upon receipt of a data output demand signal from the host computer (not indicated in the Figure) and the host computer first stores the delivered image data in the buffer memory and then processes the image by taking out the image data stored in the buffer memory one after another. Here, if the buffer memory becomes full of image data (hereinafter called the Buffer Full State) while the host computer is processing the image (hereinafter called the Busy state), the host computer sends a data output stop signal to the said image scanner to avoid overflow of delivered data, and thereby suspends temporarily the reading. When the said host computer is liberated from the Busy state and buffer memory becomes empty of image data (hereinafter called Buffer Empty), the host computer sends the data output demand signal to the image scanner and the image scanner resumes reading.
When such interruption/resumption is done while reading of one page of the original, the problem may occur that the images before and after the point of interruption of reading do not properly match each other and the picture quality deteriorates.
Generally speaking, a read sensor such as a CCD image sensor is driven by pulses of a fixed interval, from the necessity to keep the reading output uniform. On the other hand, in order to ensure a smooth start/stop of the pulse motor which causes light source, mirror etc. to scan, it is indispensable to have a slow up/slow down control. Therefore, in order to properly match the images before and after interruption/resumption of reading, it is necessary to simultaneously satisfy the aforementioned two requirements which are fundamentally contradictory with each other.
To solve this problem, with the conventional image reading apparatus, the exposure scanning system is caused to retreat by n lines after interruption if n lines are necessary for slow up and when the system is resumed, slow up is made during this n line delay and image reading is begun from the (n+1)th line, thus properly matching the image to the image before the interruption, but such arrangement causes a substantial delay of processing speed.
In a case where the pulse motor must be started and stopped by the specified driving frequency pluses in the state where scanning is being performed under stationary conditions (hereinafter called the Stationary State), the rotation frequency of the pulse motor under the Stationary State must be set within the range where the performance can follow-up the driving frequency of pulse motor, that is, within the range of the auto-start up frequency to meet the torque necessary for performing the scanning. In addition, it is necessary to provide a sufficient torque margin taking into account the variation of load due to change of environmental temperature and decrease of torque of the motor itself. Therefore as illustrated in FIG. 2, when the number of pulses required for forwarding the motor by 1 line is N and the cycle of reading interval of the CCD image sensor is T, the driving frequency of pulse motor is N/T (pps) and when the driving load is t.sub.1 (Kg.multidot.cm) and the pull-in torque for driving frequency N/T (pps) is t.sub.2 (Kg.multidot.cm), the motor start up torque t.sub.s (Kg.multidot.cm) is so set as to satisfy t.sub.1 &lt; t.sub.s &lt;t.sub.2 and t.sub.s -t.sub.1 under this state becomes the torque margin Mt. This torque margin Mt should theoretically be smaller at the Stationary State than at start up time but it is in reality by far larger at the Stationary State than at start-up time. Excessive torque becomes the cause of vibration of the motor itself or the scanning system and in particular at the time of stopping, it falls into a state as if an emergency brake has been engaged and as a result clattering of a motor itself and the scanning system due to vibration becomes substantial. In other words, when interruption/resumption performance is made during reading, the image output of the original as shown in FIG. 3A becomes as shown in FIG. 3B and the images before and after the point of interruption/resumption do not connect properly due to the aforesaid vibration and thus picture quality deteriorates.